The present invention relates generally to continuous food processing apparatus and more specifically to monitoring of food processes and apparatus for processing particulate food products.
It has been well known in batch cooking processes that it is important to heat food to a certain temperature level and maintain the food at that temperature level to eliminate certain potentially harmful bacterial contamination. After such processing, the food may be sealed from the atmosphere in a can or jar and preserved at room temperature for consumption months after preparation. As consumer demand for precooked foods has increased, the manufacturers of such foods have developed new processes and packages. Included among these precooked foods are various pasta products which are precooked and packaged in plastic bags, cups and the like, with preparation involving only heating for a relatively short time. In order to reduce manufacturing costs, it has been common to cook such food products in continuous processes in which the products are cooked in a continuously flowing stream never being exposed to the atmosphere between the cooking and final sealing into a leak-tight package.
While it is relatively easy to operate a batch process to assure cooking times at particular temperatures, the task becomes more difficult in a continuous process because of the changing conditions to which each product is subjected. One approach has been to overcook all of the products going through this continuous process in order to guarantee that all processed products receive the minimum necessary amount of cooking. This approach is considered unsatisfactory since the product is less appealing to the consumer and the additional processing adds cost to the product. Accordingly, it is often of interest to determine accurately the residence time of a food product within a cooking process to ascertain that the product has received minimum required amount of heat to destroy harmful bacteria.
In the processing of particulate food products such as pasta, including macaroni, it is often important to have information on the residence time of the particulate food products in order to control the process properly. The determination of residence time in processes in which products are conveyed in a fixed order one after another is relatively simple, being dependent on a conveyor speed or the rate of flow through the process. However, there are many processes involving flowing streams of particulate products wherein the individual pieces may have different residence times in the process as a consequence of the turbulence in the flow or of the variation in stream velocity at one point as compared to another such turbulence and variations in velocity result in some pieces leapfrogging other pieces and moving through the process much faster than others, producing a wide variation in residence times for pieces moving through the same process. In such processes involving streams of particulate products, attempts have been made to measure residence time by visually monitoring the passage of identifiable specimens mixed in with the particulate products being processed. This approach is not satisfactory since it is time consuming and subject to error at best, and is completely impractical and unusable in many processes. In most cooking processes involving streams of food particles, it is impossible to make meaningful measurements of residence times of individual particles based on visual monitoring.
Another currently used method of measuring residence time is to place magnetized particles in the stream that may be sensed by coils as they pass monitoring stations. This method provides no means of identifying specific samples and leads to possible mistakes due to the leapfrog effect of the magnetized particles. There are further problems in providing magnetizable particles to be monitored.
A third possible method of checking residence time in a cooling process involves introducing into the process spores of the bacteria which is to be killed in the cooking process. The output of the process is then sampled and checked to determine if the spores have been destroyed. This approach to verifying the effectiveness of a process in destroying bacteria is discussed at length in an article entitled "Safety Considerations in Establishing Aseptic Processes for Low-Acid Foods Containing Particulates" appearing in the March 1989 issue of Food Technology. While the use of bacteria spores as discussed in this article is a useful tool in evaluating the effectiveness and safety of a process, it has many shortcomings and is complicated to implement effectively. In addition, it provides no direct measurement of residence time of particles in the process and how such residence times vary.
In the cooking of pasta for sale as a precooked food, the pasta is typically circulated through a pipe having a diameter of 2 or 3 inches, with the pasta being transported in an liquid solution. Heaters are disposed along the pipe; and after bringing the slurry of the pasta up to a cooking temperature, it continues to be circulated through a tortuous conduit where the slurry maintains its temperature and continues to be cooked. Thereafter, there are cooling stages in which the temperature of the slurry is lowered and then the product is inserted into sterilized packages without ever being exposed to the atmosphere where it might pick up bacterial contamination. The problem of measuring the residence time of pasta in such a process is very substantial.
The flow of the pasta through the pipe or conduit is difficult to predict, varying between so-called "plug flow", in which the mass moves at a uniform velocity across the diameter of the pipe, and normal liquid flow pattern, in which the flow at the center is the maximum velocity decreasing toward the walls of the pipe where the velocity is the minimum. It is useful to determine not only the residence time of the fastest moving food particle in the process, but also the spread or range of residence times. The fastest or shortest residence time allows one to adapt the process to provide the minimum acceptable cooking, by a combination of time and temperature level, to satisfy the requirements with respect to elimination of harmful bacteria. By determining the spread or distribution of residence times between the shortest residence time and the longest allows for the evaluation of the process to consider the amount of over-processing to which the longer residence food products are subjected. There are no methods or apparatus currently available that would provide residence times on food particles, such as pieces of pasta, passing through a continuous cooking process as described above.
The various types of bacteria which the cooking process must be designed to kill are salmonella, clostridium botulinum and other mesophilic and thermophilic spore-forming spoilage organisms. The risk from these harmful bacteria is increased when the precooked foods are low acid foods such as pasta. The nature of this risk is discussed in the above cited article from Food Technology as well as an article entitled "Validation of Aseptic Processing and Packaging" appearing at pages 119-122 in the December 1990 issue of Food Technology. The article from the March 1989 issue of Food Technology discusses the importance of determining residence times in such cooking processes. The following statements are quoted from page 119 of the March 1989 article on Safety Considerations In Establishing Aseptic Processes for Low-Acid Foods Containing Particulates:
Any process establishment data for particulate processes must include detailed studies on particle residence times in both the heat exchanger (if applicable) and the holding tube of the commercial system. Tests should be conducted using the actual food product flowing steadily through the system.
From the foregoing, it is evident that there are major health and safety risks involved in bacterial contamination of low acid foods such as pasta that are cooked in a continuous process, and that accurate measurement of particle residence times is critically important to the elimination of such risks.
There have been various attempts at measuring flow velocity of a stream by sensing the rate of movement of an element that has been tagged or marked in some way. The King et al. U.S. Pat. No. 4,627,987 teaches the use of a magnetic field imposed on a flowing material as a tag to sense the rate of movement of a stream of material. The Lew U.S. Pat. No. 4,627,294, shows the use of an eddy generator to create a turbulence the progress of which is used to measure the velocity of the stream through a conduit.
Similar approaches for measuring velocity of fluid flow in pipelines are shown in Metcalf U.S. Pat. No. 2,631,242, which uses a radioactive isotope as a trace means, and Klinzing et al. U.S. Pat. No. 5,022,274, which charges particles in an air stream and senses the transit time of the charged particles to determine velocity. None of these approaches would be useful to sense the residence time of individual pieces in a stream of particulate products.
There are teachings in the prior art as to the use of transponder sensing means to locate or follow the progress of articles through a process or along a route of travel. In this connection, the patents to Hesser U.S. Pat. No. 4,588,880, Mori U.S. Pat. No. 3,366,952, and Works U.S. Pat. No. 3,745,569 are noted of interest.
At the present time, Texas Instruments Incorporated is marketing a system including miniature transponders with sensing and reading means designed for manufacturing and shipping applications for object tracking, for security and for animal identification. In connection with manufacturing applications, Texas Instruments suggests that its transponders may be placed on or within a product, product package or shipping container to provide data for process automation and statistical process control.
Many food processes and particularly food cooking processes are very dependent on the residence time of the product in each of a number of stages of the process and on the temperatures to which the product is subjected during each of these stages of the process. In the cooking of particulate material being processed in a continuous stream having turbulent conditions and particles in the stream moving at different velocities, there is no known method or apparatus for tracking a number of individual particles to provide a range of residence times in the various stages of the process. This lack of means for determining residence times for the various particles in the stream in various stages of the process requires that the entire stream be over-processed to assure that a certain minimum processing has been achieved for all particles in the stream. If a more accurate means of measuring the range of residence time that would be expected in the various stages of the process were available, it would be possible to avoid over-processing and to assure accurate processing in a minimum amount of time. There are some types of cooking processes which require that food be cooked for a certain period of time at a certain temperature to meet established health and safety standards in order to destroy harmful bacteria as discussed above. The cooking time may be shortened to some extent by increasing the temperature. In order to meet these requirements, the food processor must maintain records of the operating times and temperatures for the process and be in a position to show that his process was operating within the established standards.
Accordingly, it is an object of the present invention to provide an improved means for determining residence time of particulate material in a process involving a continuous stream of such particulate material.